Commonly employed fabrication techniques for displays and semiconductor electronic devices involve several imaging steps. A color filter substrate coated with a resist or other sensitive material is exposed to radiation through a photo-tool mask to effect some change. By nature these fabrication processes involve a large number of separate steps, each step commonly having a finite risk of failure, thus reducing the overall process yield and increasing the cost of the finished article. A specific example is the fabrication of color filters for flat panel displays also known as a liquid crystal displays. Color filter fabrication can be a very expensive process because of the high cost of materials and low process yield. Traditional photolithographic processing involves applying color resist materials to a substrate using a coating technique such as spin coating, slit and spin or spin-less coating. The material is then exposed via a photo-tool mask followed by a development process.
Direct imaging has been proposed for use in the fabrication of displays and in particular color filters. U.S. Pat. No. 4,965,242 to DeBoer et al., for example, describes a dye transfer process for making a color filter element. A color filter substrate, also known as a dye-receiving element, is overlaid with a dye donor element (also known as a color transcription film) that is then imagewise heated to selectively transfer the dye or pigment from the donor to the receiver. The preferred method of imagewise heating is by means of a laser head preferably comprising a plurality of laser beams. Diode lasers are particularly preferred for their ease of modulation, low cost and small size. It should be noted that the term “dye transfer process” is not, as its name implies, limited to the image-wise transfer of dyes. The dye transfer process can also include the image-wise transfer of dye donors coated with pigments and similar type colorant compositions.
Direct imaging systems typically employ laser heads with hundreds of individually modulated beams in parallel to reduce the time taken to complete the image. Each of the beams is modulated to create a corresponding laser pixel during the imaging process. U.S. Pat. No. 6,146,792 to Blanchet-Fincher et al., for example, describes the production of a durable image on a receiver element, such as a color filter. The laser head suggested in the examples consists of thirty-two 830 nm laser diodes, each with approximately 90 mW of single-mode output. Imaging heads with even more channels, or laser pixels, are now commonly available, exemplified by the SQUAREspot® thermal imaging head manufactured by Creo Inc. of Burnaby, British Columbia, Canada. These imaging heads are available with up to 240 independent imaging channels, each channel having power in excess of 100 mW. The image is written in a series of bands or swaths created by the plurality of laser beams, which are closely abutted to form a continuous image. (It should be noted that the terms “channel” and “laser pixel” are used interchangeably herein.)
One problem with multi-channel imaging systems is that it is extremely difficult to ensure that all channels have identical imaging characteristics. Channel-to-channel variations with respect to power, beam size, beam shape and focus all contribute to the production of a common imaging artifact known as “banding.” Banding is often particularly prominent in the area between successively imaged swaths. Each swath is defined by a corresponding image having a beginning end line and an ending end line. Consequently, banding occurs primarily because the terminating end line of the last scanned swath and the beginning end line of the next scanned swath are usually written by channels at opposite distal ends of the swath and are more likely to have differing imaging characteristics. A gradual increase in spot size from channel to channel may not be visible within the swath, but when a swath is abutted with another swath the discontinuity at the swath boundary may become quite pronounced. These pronounced swath boundary discontinuities may lead to objectionable visual artifacts when viewing a color filter that has been produced by a dye transfer method.
Banding is a common problem in multi-channel imaging and may be reduced by careful alignment and calibration of the imaging head. However, thermal and mechanical drifts may result in banding re-appearing after some time. Accordingly, there remains a need for imaging methods that lessen the visibility of banding, particularly at swath boundaries.